1. Field of the Invention
The present invention relates to a digital data modulation circuit for converting digital data into a modulated signal before the digital data is recorded in a recording medium having a DC cutoff characteristic or the digital data is transmitted through a transmission path having a DC cutoff characteristic and a digital data demodulation circuit for inversely converting the modulated signal into the original digital data after the digital data is reproduced from the recording medium or the digital data is transmitted through the transmission path.
2. Description of the Related Art
When digital data is recorded in a recording medium such as a magnetic recording medium having a DC cutoff characteristic, in which a DC component cannot be recorded, a DC component is lost upon recording. Therefore, the waveform of the reproduced data does not completely coincide with that of the original data, and a data reproduction error occurs due to the DC component lost upon recording. For this reason, several methods have been proposed or realized in order to suppress a DC component of data to be recorded by performing modulation of the data, thereby reducing such an error.
For example, Japanese Patent Disclosure (Kokai) No. 58-75950 discloses an encoding method for binary data. This method relates to data having a correlation between items of data such as a digital picture signal. In this method, mapping between "0" and "1" bits is improved when n-bit data is converted into n-bit data so that the number of "0" bits becomes substantially equal to that of "1" bits, thereby suppressing a DC component. The most serious problem of this method is that the method cannot be applied to data not having a correlation.
A system such as "8/10 MODULATION CODES FOR DIGITAL MAGNETIC RECORDING" described in IEEE TRANSACTIONS ON MAGNETICS, Vol. MAG-22, No. 5, SEPTEMBER 1986 converts m-bit data into n-bit data (m&lt;n) and suppresses a DC component by utilizing the redundancy of the converted data. This system can suppress a DC component regardless of a statistical property of data such as a correlation. In this system, however, a bit rate of data is undesirably increased after conversion.
In addition, technical report of "The institute of Television Engineers of Japan" ICS81-10 describes "A NEW DC-SUPPRESSED CHANNEL CODING FOR DPCM". This coding system improves mapping by utilizing a distribution and a correlation of differential values when a picture signal is subjected to DPCM and transmitted, thereby suppressing a DC component. This method cannot be applied except for the DPCM.
As described above, according to the conventional techniques, in order to suppress a DC component without increasing a bit rate, a statistical property, i.e., the distribution and the correlation, of data must be predictable and usable. In order to suppress a DC component regardless of a statistical property of data, a bit rate must be increased (normally, about 10% or more). Therefore, operation efficiency of a recording medium or transmission medium is decreased.
In the conventional techniques, therefore, data can be recorded or transmitted after suppressing a DC component without increasing a bit rate only when a statistical property of the data is predictable and a distribution, a correlation, and the like of the data are predetermined. When a statistical property of data is unknown or unusable although it is predictable, a bit rate must be increased by normally about 10% in order to suppress a DC component. For this reason, in order not to decrease recording or transmission efficiency, data to be recorded or transmitted must be strictly limited. As a result, an application range of each of the conventional systems is significantly narrow.